Historically, power companies use large synchronous generators to provide alternating current (AC) power to the power grid. Traditionally, these generators are 3-phase sinusoidal AC designed for a 3-phase AC grid. These generators, which are often powered by steam or gas turbines fueled by coal, oil, natural gas, or nuclear power, are typically operated at a constant speed because the frequency of electrical network is fixed. Even for the applications where DC power is required, a 3-phase AC generator is used along with a 3-phase rectifier to convert the AC power to DC power. If a simple passive diode rectifier is used, the rectifier introduces significant harmonic current and causes rotor losses and torque ripple. In addition, a simple rectifier does not have high fidelity control capability of regulating torque and voltage.
As interest in alternative sources of energy increases, interest in wind power has risen. Wind generators, however, must operate under varying wind conditions and thus suffer large variations in speed. In order to maximize power output of a wind generator across a range of operating speeds, the torque of the generator is regulated or adjusted depending on the present wind/rotor speed, e.g., by adjusting coil voltages and currents.
In conventional wind generators, the control system performs maximum power point tracking (MPPT) by regulating the electromagnetic torque through pulse width modulation (PWM) switching. However, this requires active rectifiers that are more costly, less efficient, and less reliable than the simple diode rectifiers that can be used in constant-speed conventional generators. In addition, simple diode or thyristor rectifiers do not have sufficient bandwidth control capability to regulate torque or DC voltage under the varying conditions in which wind generators typically operate.
Multiphase winding electric machines allow passive rectifiers to be used without introducing the torque ripple and additional rotor losses that are commonly present in simple diode or thyristor rectifiers. However, it is difficult to control torque and voltage with passive rectifiers, and for this reason their use is typically limited to field wound synchronous machines and fixed speed operation. Passive rectification is not well suited for use with wind generators. One reason is that wind generators have a large speed variation, e.g., from 50% to 100% of nominal speed. As a result, the output voltage in the low speed range is too low and needs to be boosted to nominal voltage so that a high-fidelity grid-side inverter control can be utilized. Another reason is that typical wind generators are permanent magnet generators, which means that the rotor flux cannot be changed. As a result, it is difficult to control torque using passive rectifiers. Active rectification is needed, which requires some form of control input, usually provided by a DC-DC converter.
There are disadvantages to active rectification, however. A common approach is to add a DC-DC converter to regulate torque and DC voltage, but conventional approaches require that the DC-DC converter be rated at full power, i.e., that the DC-DC converter not only operate correctly when the wind speed—and thus the output voltage of the wind generator—is low, but also be able to operate correctly when the wind speed and output voltage of the wind generator is high. Such DC-DC converters tend to be very complicated and correspondingly expensive.
Accordingly, in light of these disadvantages, there exists a need for methods and systems for electrical DC generation suitable for generators that operate at varying speeds.